One of the most desirable characteristics of ballistic armor is having the lowest possible weight of armor material to provide the required protection against the maximum expected threat level. For vehicles and aircraft, reduced armor weight means extended range, increased payload, lower fuel consumption, and reduced maintenance costs. For personnel, lower armor weight means reduced fatigue and increased comfort, which translates to improved performance.
Nonoxide armor ceramics that are currently being manufactured, such as boron carbide, silicon carbide, and silicon nitride, are difficult to density to high density to form a useful material. Typically, very high temperatures and applied pressure are required to consolidate these materials and, even under these conditions, it is often necessary to add chemicals to the ceramic composition to aid in densification.
The lowest weight armor ceramic is boron carbide (B4C). Most composition and processing methods used to produce commercially available boron carbide armor tiles are proprietary and, therefore, not available for comparison.
Examples of methods for densifying boron carbide are given in U.S. Pat. No. 4,195,066 which describes the introduction of carbon additives for pressureless sintering of boron carbide. From 0.5 to 10% by weight amorphous carbon is added to promote sintering of boron carbide powder having a particle size≦1 μm. Typically, for most ceramic materials, a fine powder particle size enhances consolidation and sintered density.
Thevenot, in “Sintering of boron carbide and boron carbide-silicon carbide two-phase materials and their properties,” Journal of Nuclear Materials, v 152, n 2-3, May 1988, p 154-62, discloses the use of polymeric precursor additions, e.g. polycarbosilane plus phenolic resin, to obtain boron carbide ceramics with a density>92% and containing approximately 5% silicon carbide by weight.
The use of alumina as a sintering aid is described by Lee in “Pressureless sintering and related reaction phenomena of Al2O3-doped B4C,” Journal of Materials Science, v 27, n 23, 1 Dec. 1992, p 6335-40. Boron carbide containing 3% alumina by weight is sintered to 96% of theoretical density at 2150° C.
Addition of silicon carbide and/or titanium carbide to boron carbide powder is described in U.S. Pat. Nos. 5,418,196 and 5,637,269 to aid in the sintering of a boron carbide body by hot pressing at up to 2300° C.
U.S. Pat. No. 5,505,899 teaches the addition of one or more metal monocarbides of the elements Ti, Zr, Hf, V, Nb, and Ta in an amount corresponding to from 2 to 6% by weight free carbon to a boron carbide composition to promote sintering at from 2100° C. to 2250° C.
U.S. Pat. No. 5,720,910 describes the use of titania and carbon additions to boron carbide powder to enhance densification at from 1900° C. to 2100° C.
In addition to sintering methods which require the use of additives that are incorporated in the boron carbide starting powder, other approaches for achieving dense boron carbide bodies entail the use of alternative processing means.
Kalandadze, et al. in “Sintering of boron and boron carbide,” Journal of Solid State Chemistry, v 154, n 1, October 2000, p 194-8, describe the use of explosive compression of powder compacts of boron and boron carbide to increase sintered density.
The use of a processing method called “plasma pressure compaction” is evaluated by Klotz, et al. in “Characterization of boron carbide consolidation by the plasma pressure compaction (P2C) method in air,” Ceramic Engineering and Science Proceedings, v 22, n 4, 2001, p27-34, where the boron carbide powder is subjected to a pulsed DC voltage followed by an applied uniaxial pressure and high-current, continuous DC voltage to density the material.
Shul'Zhenko, et al. describes an ultra-high pressure boron carbide consolidation process in “Formation of polycrystalline boron carbide B4C with elevated fracture toughness,” Powder Metallurgy and Metal Ceramics, v 44, n 1-2, January 2005, p 75-83, where the sintering takes place under a pressure of 5.5 GPa (about 798,000 psi) at a processing temperature of 2200 K (about 1927 C).
The composition and processing history of boron carbide ceramics will determine the physical properties of the material, however, the characteristics of a material that will result in superior ballistic impact resistance are not well understood and armor material performance is ultimately rated by field testing against actual ballistic threats.